1. Field of the Invention
This invention provides a method and apparatus for use with a liquid metal-to-water tube type heat exchanger, for detecting the location of a defective tube and the precise position in the tube of the defect.
2. Description of the Prior Art
A major concern in the design and operation of liquid metal cooled nuclear reactors is the possibility of leakage at a steam generator allowing liquid metal in one system to mix with the water and steam mixture in the steam system. The steam generator to which this invention relates is simply a heat exchanger in which heat is transferred from the liquid metal system to water in the steam system. The violent liquid metal-water reaction resulting from a leak in a steam generator causes rapid corrosion and erosion of metal tubes adjacent to the leak. There is also the possibility of damage to the entire steam generator and other portions of the system to which the steam generator is directly connected by means of shock wave propagation in the event of a significantly large mixture of liquid metal and water. Hence it is imperative that any such leaks be located and isolated if propagation of damage is to be avoided.
Several on-line monitoring techniques exist which grossly detect a leak between the water-steam mixture and liquid metal systems. Most of these techniques are based upon the detection of an increase in hydrogen concentration or an increase in oxygen concentration in the liquid metal system. If sufficient hydrogen or oxygen or other monitoring is utilized these techniques can isolate the specific steam generator in which a leak is occurring. However, none of these on-line techniques can detect the specific tube, or group of tubes, or the specific position of a defect, through which leakage occurs. The sensitivity of these systems is in the range of 10.sup.-4 to 10.sup.-5 pounds of water per second leak rate. Upon detection of a leak the reactor plant must be shut down, the specific leaking tube or tubes located, and a repair made. The generally accepted repair mode is to seal the leaking tube by one of many alternate means, one of such means being by explosive welding, such that the fluid designed to flow through the tubes is excluded from entering the tubes through the normal inlet path. With the emphasis placed upon nuclear generating availability and the high cost penalty associated with down time, any means by which down time can be minimized vitally serves the interest of the electric utility industry. This invention provides not only a faster means for locating a specific tube leak, but also provides a less expensive means compared to the presently available alternative techniques. Most of these alternative techniques have been based upon eddy current flaw detection principles, which necessarily involve traversing a probe through each tube. This invention can utilize a traversing probe, but is not necessarily so limited. One reason for the use of eddy current techniques has been the chemical reaction of water and a liquid metal at the leak location when the tube is cooled and these reactants, water and liquid metal, are removed from the vicinity of the leak. Under these conditions it is found that the leak becomes self-healing in the sense that reaction products will solidify and plug the leak. With an abundance of moisture available, the reaction products in a system utilizing liquid sodium coolant include NaOH, Na.sub.2 O, NaO, NaH, and hydrogen. As a result of this self-healing effect, leak location detection methods could not depend on the existence of an open path through the wall of the tube. This invention, on the other hand, may specifically utilize the reaction products formed in the cool down process as an element in the leak detection process, as hereafter specifically described.